In the specific field of application of this invention, it is common practice to provide high-voltage resistors with a horizontal geometry which is essentially spiral-shaped and formed on a semiconductor substrate having a predetermined, opposite polarity from that of the resistor.
One such construction is known, for instance, from U.S. Pat. No. 4,792,840 and allows the resistor to so distribute the equipotential lines across the substrate surface as to hold the difference of potential at the junction points between the resistor and the substrate surface below the junction breakdown voltage. The potential difference varies linearly toward the spiral center, which allows the resistor to withstand high voltages of up to 400 Volts.
While being beneficial from this point of view, said prior approach still has a serious drawback.
When any two regions of the resistor are taken which lie close to each other, i.e., regions of two adjacent turns or convolutions, one of them is bound to be at a lower potential than the other. As a result, the positive charges present in the oxide layer that covers the spiral will be pulled toward the metallic contact associated with the lower potential region.
The build-up of positive charges gradually forms a conductive layer proper, referred to in the art as "phantom layer," which extends toward the higher potential region.
This phantom layer, being additionally connected to a negative potential present at the metallic contact, establishes conduction between the aforesaid two and underlying regions at different potentials.
Briefly stated, the phantom layer functions as if it were the gate of a MOS transistor, with the two regions at different potentials corresponding to the source and drain of the transistor.
This reflects in deep alteration of the spiral resistor characteristics, and deterioration of its reliability.